Sensory perception plays an increasingly important role in robotics. By means of sensory perception, it is desirable to obtain as much information about the environment as possible to ensure that robots respond optimally. In robotics, the most frequent tasks are gripping, positioning and object relocation. The approaches used so far were indeed feasible in a well-modelled environment, where the position and orientation of the object were known. The robotic arms and gripping devices becoming increasingly widespread which fact led necessarily to the expectation of applying them in as broad a range as possible, and also in an undefined environment. A future goal of robotics is that robots should have similar capabilities to human beings and provide assistance for them. To this end, it is indispensable that robots should handle objects (for example door handles, cups, buttons, glasses, etc.) relating to a task with a skill and safety similar to humans.
In the course of interaction with various objects, the primary information is generated by tactile perception. This interaction may carry important information not only on the fingers, but also on the whole surface of the robot, and therefore extension of sensors to the complete surface of the robot, the creation of a kind of artificial skin, is a further goal for researchers.
An overview about generally applied sensors and pressure sensor devices is given by J. Tegin and J. Wikander, Tactile sensing in intelligent robotic manipulation—a review, Industrial Robot: an International Journal, Vol. 32, No. 1, pp. 64-70 (2005). In summary, it can be stated that the current sensor devices have rigid surface and structure. The contact established between the object and the sensor device is point-like in most cases, especially in the case of rigid objects. It is a great disadvantage of the point-like contact that little information can be obtained about the given object and its surface, and stability of gripping is also reduced.
In the case of pressure sensor devices having elastic surface, as a result of applying a gripping force, the surface of the sensor device fits to the surface of the object, thereby increases the stability of the grip and information about the surface and material characteristics of the object can be collected from larger surface. In many prior art solutions, it was attempted to cover rigid structure sensor devices with a flexible material.
Another important task, in which these approaches are less beneficial, is determining the direction and location of pressure. Sensing the direction of tactile perception (from the forces exerted on one finger for example) is especially important in determining slipping and the characteristics of the surface.
In US 2010/0253650 A1, such an optical pressure sensor device is disclosed which has a flexible dome, i.e. cover layer, and it has furthermore a light reflecting layer to reflect the signal of light emitting elements to light detecting elements. It is a disadvantage of this approach that the useful surface of the sensor device, on which it can detect with an appropriate accuracy the components of forces acting thereon, is limited by the emission angle of the light emitting element and by the angle of vision of the light detecting elements. A further disadvantage of the approach is that as a result of easy compressibility, it is only able to measure the force within a relatively small value range.
In US 20091315989 A1 a sensor with elastomeric filler material is disclosed. In US 2003/0173708 A1 optical shield is applied between respective light sources and light detectors.
In U.S. Pat. No. 4,704,909 a pressure sensor device having also optical embodiments is disclosed, in which an elastic ring is applied under a rigid surface which is actually a flat covering of the sensor device.
In U.S. Pat. Nos. 4,635,479 and 4,747,313 a pressure sensor device having a dome-shaped pressure sensor surface made from a rigid material is disclosed. In U.S. Pat. No. 4,405,197 an optical principle based pressure sensor device having an elastic pressure sensor surface is disclosed.
A number of other three-dimensional sensor devices designed for determining the components of pressure force are known. Most of them are based on MEMS (Micro-Electro-Mechanical Systems) technology, such as the sensor device disclosed in US 2009/0320611 A1. In general, these sensor devices are highly susceptible to damage and have a small sensor surface, which practically makes them suitable for measuring forces acting point-like.
Another well-known and broadly used solution is applying optical pressure measurement by CCD or CMOS cameras. The basic principle of operation in this case is that various markers well-detectable by camera are placed on an elastic material above the camera, and the distance of the markers from the camera changes as a result of material deformation (external force). The change in the distance and hence the rate of surface deformation can be detected on the basis of the camera picture, i.e. the forces acting on the surface can be calculated. Such a realization is disclosed in the following study: P. Lang, Optical tactile sensors for medical palpation, The Thirty-Fourth London District Science and Technology Conference, pp. 1-5, (2004). The disadvantages of these realizations are the large size and the high calculation complexity.
Sensor devices may also be based on magnetic principles. According to the publication disclosing this approach (E. Torres-dara, I. Vasilescu, and R. Coral, A soft touch: Compliant Tactile Sensors for Sensitive Manipulation, Technical Reports, Massachusetts Institute of Technology, Computer Science and Artificial Intelligence Laboratory, 1 Mar. 2006), a magnet is fixed to the centre of an elastic dome above four Hall-sensors placed in one plane. With the deformation of the dome, the location of the magnet respect to the Hall-sensors changes which can be measured by a variation of the magnetic field, but the gripping of metal or magnetic objects may disturb the measurement. In the above, and also in the associated US 2010/0155579 A1 document, the optical realization of the discussed sensor device is mentioned. In the optical realization, a hollow dome reflecting the optical signals is applied above the optical signal emitting and detecting elements, but this limits the loadability and miniaturisation of the sensor device. A further disadvantage of this solution is that it does not prevent the ingress of external light into the sensor device.
The fact that the grippers only comprise a few sensor devices in robotics, especially located at the ends of fingers, is not a consequence of the quality of sensors. It is a huge problem to design the wiring of sensor devices and sensors, and with the increase in the number of sensors, the signals to be guided away and processed is growing, and this also increase the size of signal processing electronics. The application of sensor matrices (with row and column detection) assists the solving of this problem, but even in the case of one fingertip, when using a 8×8 sensor field, the application of at least 16 wires is necessary.
It is a common disadvantage of some of the above solutions that the angle of vision of applied light emitting elements and light detecting elements make a constraint on the size of the useful surface of the sensor device. The common disadvantage of a further part of solutions mentioned above is that their measuring range is heavily restricted, and because of their mechanical design, they are unsuitable for the measurement of force varying within a broad range.
In view of the known solutions the need has arisen to provide a sensor device being adapted to sense the impacts generated in various ways, e.g. as a result of force or pressure, with the possibility of implementation in a broad range of dimensions, having as large a surface as possible in relation to the sensor dimensions to detect the pressure vector and it is able to measure force and pressure, respectively, in the widest possible value range.